A configuration of flat conductive circuits on a substrate of flexible insulating material is well known. The circuits are fabricated on the surface of the substrate by a manufacturing process that is a subtractive process.
According to a typical subtractive process, an insulating substrate is laminated with copper foil covering the substrate making a three layer structure of substrate, adhesive and foil. The foil is covered by a photosensitive material that is resistant to a metal enchant. The photosensitive material is photoexposed by illumination, to sensitize the material for removal when washed. During photoexposure of the photosensitive material, an opaque mask casts a shadow over portions of the photosensitive material. The mask shadow is configured in a pattern of an electrical circuit. Thereafter, when the photosensitive material is washed, the unshadowed material is removed, leaving the shadowed material remaining in a pattern of a desired circuit on the substrate. The metal is then etched, hence, subtracted from the substrate, except where the shadowed material resists the enchant. Thereafter the shadowed material is cleaned from the metal that remains as the electrical circuit adhered to the substrate. The adhesive layer is a disadvantage because of mechanical and thermal instability of adhesives. In the example of a fine line circuit, the adhesive thickness is 0.001 inches and the circuit is 0.0007 inches thick by 0.005 inches wide, which cause both circuit positional tolerance and contact force problems. Customers want an adhesiveless solution.
It has been recognized that medical instruments such as esphogael electrodes require special printed circuit characteristics. One such electrode is disclosed for example in U.S. Pat. No. 5,069,215. Such a printed circuit characteristic is being able to withstand the daily medical applications.
The electrical circuit disclosed in the above referenced patent utilizes a silk screen method to create its circuitry. The silk screen method is a multiple step process. The process includes replacing a paper liner with a polyester carrier onto a base material. The substrate is then silkscreened with a silver ink (silver flakes or powder which is suspended in an organic media that is cured) circuit upon a polyester substrate after the substrate had an acrylic adhesive applied. The silver is partially cured so that the silver circuit can be inspected. The silver circuit is then insulated by a silk screening process. The insulation is cured and the circuits are then resistance tested. The paper liner then replaces the polyester carrier. This method produces circuits having 80,000-130,000 .ANG. thick. This circuit thickness is too much for medical instrumentation because it is subject to cracking and silver is also deleterious in medical applications.
The circuit pattern required is a 12 lead circuit with ability to sustain a 5.0 MHz transducer mounted on the tip of medical instruments, like a gastroscope. Moreover, the circuit pattern has to carry an input impedance greater than 500 kohms at 10 Hz for a preamplifier along with a differential input gain of 220 Hz high-pass filters. The printed circuit must also deliver a constant current delivery continuously variable from 0-40 ma and a load impedance at a constant current at impedance up to 4000 ohms at 20 ma.
There is a need to create a circuit that is efficiently produced, have longer use due to the strains of medical applications and which permits a 75.OMEGA. per circuit resistance drop.
An additive process for disposable medical circuits would apply metal solely where needed on the substrate economically.